1. Field of the Invention (Technical Field)
This invention relates to apparatuses for lengthening bones, primarily for the lengthening of the mandible by distraction osteogenesis.
2. Background Art
There are a variety of conditions characterized by asymmetric or unusually small mandibles. These include hemifacial microsomia, Treacher-Collins syndrome, lateral facial cleft, Pierre-Robin anomalies, post-traumatic growth asymmetries, and deformities resulting from traumatic injury. The incidence rate for hemifacial microsomia is approximately one in 5,000 births.
The preferred method for treatment is by distraction osteogenesis, in which a controlled fracture of the mandible is separated over a period of time, after the callus forms, allowing new bone to fill in the area of the stretched callus. Distraction osteogenesis of the human craniofacial skeleton enables the surgeon to gradually reposition abnormally formed bones of the face and jaw. The basic principles of distraction osteogenesis are that a surgical corticotomy or osteotomy is made and the two segments of bone are gradually moved apart at approximately 1 mm/day and then stabilized for bone consolidation.
To date, distraction of the mandible has been accomplished using external distraction devices, in which surgical pins are placed in the mandible on either side of the corticotomy or osteotomy, transiting the skin through the cheek, and a screw or rachet driven distraction device attached to the pins to provide distraction force. However, external devices for distraction which are currently available are bulky, and because of inherent limitations, have significantly limited the use of this surgical method. In the younger child and infant, which have relatively soft and small bones, conventional external distraction devices frequently cannot be utilized. In any patient treated using external distraction, significant scarring results from the surgical pins which transect the check, with tears and infection resulting from wound stretching in the gradual separation of the bisected jaw. In addition, external distraction lacks adequate control, in part because distraction frequently flexes the pins or causes the pins to migrate through the bone; is readily subject to dislocation, particularly with younger patients; and has a socially unacceptable appearance, giving rise to poor patient compliance.
Distraction osteogenesis has substantial advantages over other methods of mandibular surgical advancement. This technique can produce unlimited quantities of living bone which can be formed along any plane of space. Because distraction osteogenesis uses local host tissue, it offers many advantages over bone grafting. Expansion of the entire functional matrix of the mandible is achieved, so that the muscle, nerves and skin are maintained intact. Distraction osteogenesis is less invasive than conventional craniofacial surgery; while the bones must be cut using either technique, with distraction they are not forcibly broken, and thus blood loss and postoperative swelling are lessened.
Distraction osteogenesis has a long history in limb lengthening and was popularized by the Soviet orthopedic surgeon, Ilizarov in the 1950's (Ilizarov G A: The principles of the Ilizarov method. Bull Hosp J Dis Orthop Inst 48:1, 1988). He developed the technique of distraction osteogenesis for gradual lengthening of the long bones to treat many congenital and acquired deformities. Distraction was first applied to the membranous facial bones by Snyder, who successfully lengthened canine mandibles with external devices (Snyder C C, Levine G A, Swanson H M, et al: Mandibular lengthening by gradual distraction: preliminary report. Plast Reconstr Surg 51:506, 1973). From numerous animal experiments, a large body of information has accumulated which details the efficacy of distraction osteogenesis in the craniofacial skeleton. In 1992, McCarthy reported on four children with craniofacial microsomia and Nagar's syndrome who underwent successful mandibular distraction with an external device (McCarthy J G, Schreiber J S, Karp N S, et al: Lengthening of the human mandible by gradual distraction. Plast Reconstr Surg 89:1, 1992). In 1994, McCarthy and Ortiz-Monasterio each reported their own successful experiences with mandibular distraction (McCarthy J G, Mandibular bone lengthening. Oper Tech Plast Reconstr Surg 1:99-104, 1994; Oritz-Monasterio F, Molina F: Mandibular distraction in hemifacial microsomia. Oper Tech Plast Reconstr Surg 1:105-112, 1994). Only recently have limited human studies been conducted utilizing a bidirectional external distractor (Klein C, Howaldt H -P: Correction of mandibular hypoplasia by means of bidirectional callus distraction. J Craniofacial Surg 7:258-266, 1996). Using external devices, there appears to be long-term improvement of mandibular morphology with minimal complications.
The most widely used prior art mandibular distraction device is unidirectional. In mandibular deformities such as hemifacial microsomia and Treacher Collins syndrome, both vertical and horizontal bone lengthening are required to correct facial asymmetries or deficient vertical facial heights. The surgical solutions utilizing distraction osteogenesis are limited by the available distraction devices. If an unidirectional distraction device is used, then the available bone and developing tooth buds tend to dictate the direction of the osteotomy cuts, the position of the fixation pins and ultimately, the vector of bone regeneration. Several surgeons have reported the use of bidirectional distractors and a two-cut approach to achieve two-dimensional control of three mandibular segments. All of these bidirectional devices are external and share the same undesirable cosmetic problems associated with other external distractors.
The advantages of an implanted device are both therapeutic and cosmetic, including less danger of dislocation by falls or bumping, greater distraction control, and no external scarring. An implantable device can also be adapted for other types of reconstruction in craniofacial surgery such as maxillary expansion, maxillary advancements, symphyseal widening and monoblock facial advancements.